Many modern electronic devices emit or are sensitive to electromagnetic interference (EMI) at high frequencies. Electromagnetic interference is understood to mean undesired conducted or radiated electrical disturbances from an electric or electronic apparatus, including transients, which can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur anywhere in the electromagnetic spectrum. Radio frequency interference (RFI) refers to disturbances in the radio frequency portion of the spectrum but often is used interchangeably with electromagnetic interference. Both electromagnetic and radio frequency interference are referred to hereafter as EMI.
Many electronic devices, for example, cell phones, computers, various radio frequency and microwave devices, among others, are sources of EMI. These devices not only are sources of EMI, but also the operation of such devices may be adversely affected by the emission of EMI from other sources. Consequently, electric or electronic apparatus susceptible to electromagnetic interference or apparatus likely to generate electromagnetic generally must be shielded in order to operate properly.
The shield generally is any metallic or electrically conductive configuration inserted between a source of EMI and a desired area of protection wherein the shield is capable of absorbing and/or reflecting the EMI. As a practical matter, such shields normally take the form of an electrically conductive housing or cabinet, which is electrically grounded. The shield, in any event, prevents both the radiation of EMI from a source and/or prevents such interference (either generated randomly or by design) from reaching a target within the shielded volume.
A shield comprising a metal cabinet often includes an opening for access to the electronics within the cabinet with a door or other removable cover closing the access opening. Any gap between the confronting, abutting or mating metal surfaces of the cabinet and closure afford an opportunity for the passage of electromagnetic interference. Gaps also interfere with electrical currents running along the surfaces of the cabinets from EMI energy which is absorbed and is being conducted to ground. The gaps reduce the efficiency of the ground conduction path and may even result in the shield becoming a secondary source of EMI leakage from gaps acting as slot antennae. Accordingly, it is common to use a conductive seal or gasket between such surfaces to block the passage of EMI.
Various configurations of gaskets have been developed to close the gaps between components of the shield. These gaskets establish as continuous a conductive path as possible across any gap that may exist, for example, between cabinet components. A common gasket employs a flexible core enclosed in a woven fabric made at least in part with conductive fibers. Examples of such fabrics are disclosed in U.S. Pat. No. 4,684,762. Another common gasket construction as disclosed, for example, in U.S. Pat. Nos. 4,857,668, and 5,597,979 has a flexible core enclosed in an electrically conductive sheath formed of a non-conducting woven or non-woven fabric. The fabric is rendered conductive by an electroless plating process wherein the fabric is dipped in a silver nitrate bath to impregnate the fabric with silver. In an alternative process, the conductive material including silver or copper may be applied by sputter deposition. After impregnation or coating with silver, the fabric is coated with a non-corrosive material to prevent the oxidation of the silver surface. Suitable coating materials applied either by electroplating or sputter deposition include a pure metal such a nickel or tin, a metal alloy such as Inconel® or Nichrome® or a carbon compound.
In addition to being conductive, the gasket also must have a degree of abrasion resistance. Resistance to abrasion is important as any wearing away of the conductive surface can result in loss of the EMI shield. Abrasion and erosion of the conductive surface occurs in response to the movement and flexing of the cabinet in which the electronic apparatus is contained and some abrasion occurs each time the door or closure is removed and replaced as may occur when the electronics are serviced.
While gaskets formed of a metalized fabric have been acceptable, the multiple steps required to manufacture such gaskets adds considerably to the cost of the gasket. Metalized films of a polymeric material also have been used as a sheathing material and in general, the manufacture of a conductive gasket from a metalized film involves fewer process steps. However, metalized films generally are not as abrasion resistant as a conductive fabric of a woven or non-woven material. In particular, when a metalized film is used as a conductive media for EMI gaskets, even low levels of abrasion that erodes the metal layer will adversely affect the surface conductivity and permit passage of EMI.
Accordingly, an object of the present invention is to provide an improved conductive gasket for use in sealing gaps between adjacent surfaces of a shielding housing for electric or electronic apparatus to isolate the electric or electronic device within the housing from EMI.
Another object of the invention is to provide an EMI gasket formed in part from a metalized polymeric film.
A further object of the present invention is to provide an abrasion resistant EMI gasket formed in part of a metalized polymeric film.
A still further object of the present invention is to provide an EMI gasket having a resilient core enclosed in an abrasion resistant metalized polymeric film.